Greens demand Assange charge answer

The Australian Greens want the federal government to reveal what it knew about secret charges United States prosecutors drew up against WikiLeaks founder Julian Assange.

Prosecutors drew up an indictment more than 12 months ago, Fairfax reported on Wednesday, citing a confidential email obtained from the private US intelligence company Stratfor.

Mr Assange is awaiting a British Supreme Court decision on his appeal against extradition to Sweden on sexual assault allegations.

He fears extradition to Stockholm would open the way for his extradition to the US on possible espionage or conspiracy charges, in retaliation for WikiLeaks's publication of thousands of leaked US classified military and diplomatic reports.

Greens Senator Scott Ludlam is demanding an explanation from the Gillard government.

"What we need to know is whether the Australian government was tipped off, or whether the prime minister read about this in The Sydney Morning Herald this morning," he told reporters in Canberra.

The Australian government needed to take "a very straight line" with the US on the issue.

"That we will not permit, and we will not tolerate his transfer to the US, to face charges that could potentially land him in prison, potentially for decades."